CD4+CD25+FoxP3+ regulatory T cells (Tregs) can suppress the cytotoxic function of activated CD8+ effector T cells, as shown in vitro and in acute brain slices ex vivo by Göbel et al. (2012) using the OT-I/ODC-OVA oligodendrocyte-directed attack model: suppression was limited by the strength of the antigen signal delivered to CD8+ T cells and, presumably, by the regulatory-to-effector T cell ratio, which the study's effector-site assays held fixed rather than varied. That study explicitly cautioned that its systems do not allow estimation of how much this parenchymal suppression contributes to the course and severity of systemic experimental CNS inflammation in vivo. No study quantifies this contribution in vivo or maps its antigen-load dependence in living animals. Adjacent in vivo results exist in lymph nodes (Mempel et al. 2006), CD4-driven EAE (Koutrolos et al. 2014), CNS viral infection (Ciurkiewicz et al. 2018), and autoimmune diabetes (Tsyklauri et al. 2023), and the ODC-OVA model has recently been established fully in vivo for CD8 entry studies (Aydin et al. 2023; Pareja et al. 2025), but none addresses Treg action at the CNS parenchymal effector site. The problem remains open.

Background

In inflammatory demyelinating disorders of the central nervous system (CNS), such as multiple sclerosis (MS), CD8+ T cells dominate the lesion infiltrate and are considered key mediators of parenchymal tissue destruction. Cytotoxic T lymphocytes (CTLs) kill target cells by releasing perforin/granzyme-containing cytotoxic granules upon recognition of cognate peptide bound to major histocompatibility complex class I (MHC I) molecules on the target cell surface. CD4+CD25+FoxP3+ regulatory T cells (Tregs), identified by the transcription factor FoxP3 together with the CD4 coreceptor and CD25 surface markers, suppress adaptive immune responses and are essential for self-tolerance. Whether Tregs can restrain an already-established CD8+ CTL attack inside a target organ, rather than only during priming in lymphoid organs, is a distinct question with direct therapeutic relevance for MS. A landmark in vivo study in lymph nodes (Mempel et al., Immunity 2006) used multiphoton intravital microscopy (imaging of fluorescently labelled cells within living animals) to show that activated Tregs reversibly suppress CTL killing of antigen-presenting target cells in the lymph node (reducing the killing rate about 6.6-fold) by selectively impairing granule exocytosis in a TGF-beta-dependent way, without affecting CTL proliferation, motility, or conjugate formation. Extending this to the CNS, Göbel et al. (Journal of Neuroinflammation 2012) used the OT-I/OT-II system, in which OT-I transgenic CD8+ T cells recognize the ovalbumin peptide OVA257-264 presented on MHC I molecules and OT-II transgenic CD4/Treg cells recognize an MHC II-restricted ovalbumin peptide, together with ODC-OVA mice, in which oligodendrocytes (the myelin-forming glial cells of the CNS) express ovalbumin as a neo-self-antigen. In cerebellar brain slices (an ex vivo model of an oligodendrocyte-directed CD8+ T cell attack, later published as a protocol by the same group, Göbel et al. 2015), activated CD4+ Tregs dampened the cytotoxicity of an ongoing OT-I effector attack: they impaired the release of cytotoxic granules (leaving CD8+ activation status unchanged), required T cell receptor-mediated activation plus costimulation to acquire suppressive capacity, and were then antigen non-specific in their suppressive effect. Crucially, the suppressive effect was limited by the strength of the antigen signal delivered to the CD8+ T cells (determined mainly by the number of peptide-loaded MHC I molecules on the target cell surface: higher OVA257-264 load on EL-4 target cells (a mouse lymphoma line used as generic CD8+ target cells) or higher (OVA257-264-loaded) MHC I expression by oligodendrocytes meant weaker relative suppression) and, presumably, by the regulatory-to-effector T cell ratio, a dependence the study itself could not directly test because its effector-site cytotoxicity assays used a fixed regulatory-to-effector ratio of 2:1. In vivo evidence that Tregs act inside the inflamed CNS exists, but for CD4-driven pathology: Koutrolos et al. (2014) showed by timed Treg depletion and two-photon microscopy in experimental autoimmune encephalomyelitis (EAE, the animal model of MS) that CNS-resident Tregs control effector T cell proliferation, cytokine production and motility, and that their depletion exacerbates disease. What no study has established is the parenchymal, CD8-specific counterpart of this result: whether and how much Treg-mediated suppression of CD8+ cytotoxicity at the CNS effector site actually shapes the clinical course and severity of systemic autoimmune CNS inflammation in vivo, and how that suppression behaves as target-cell antigen load varies under physiological inflammatory conditions.

Problem Statement

Determine, in the living organism, the extent to which CD4+CD25+FoxP3+ regulatory T cells suppress the cytotoxic effector function of CD8+ T cells at the CNS parenchymal effector site during systemic autoimmune neuroinflammation (for example in an in vivo oligodendrocyte-directed CD8 attack such as the ODC-OVA/OT-I system or a CD8-driven EAE variant), such that the contribution of this parenchymal suppression to the clinical course and severity of disease can be quantitatively estimated rather than only inferred ex vivo. Within that in vivo setting, establish how the suppression depends on the strength of the antigen signal delivered to the CD8+ effector cells, i.e., the density of cognate peptide-MHC class I complexes on the target cells, in particular whether suppression of cytotoxicity is maintained, breaks down, or is further limited under conditions of stronger or more sustained target-cell antigen presentation.

References

  1. Göbel K, Bittner S, Melzer N, Pankratz S, Dreykluft A, Schuhmann MK, Meuth SG, Wiendl H. 'CD4+ CD25+ FoxP3+ regulatory T cells suppress cytotoxicity of CD8+ effector T cells: implications for their capacity to limit inflammatory central nervous system damage at the parenchymal level.' Journal of Neuroinflammation 9:41 (2012). DOI: 10.1186/1742-2094-9-41. https://doi.org/10.1186/1742-2094-9-41
  2. Mempel TR, Pittet MJ, Khazaie K, Weninger W, Weissleder R, von Boehmer H, von Andrian UH. 'Regulatory T cells reversibly suppress cytotoxic T cell function independent of effector differentiation.' Immunity 25(1):129-141 (2006). DOI: 10.1016/j.immuni.2006.04.015. https://doi.org/10.1016/j.immuni.2006.04.015
  3. Koutrolos M, Berer K, Kawakami N, Wekerle H, Krishnamoorthy G. 'Treg cells mediate recovery from EAE by controlling effector T cell proliferation and motility in the CNS.' Acta Neuropathologica Communications 2:163 (2014). DOI: 10.1186/s40478-014-0163-1. https://doi.org/10.1186/s40478-014-0163-1
  4. Melzer N, Hicking G, Bittner S, Bobak N, Göbel K, Herrmann AM, Wiendl H, Meuth SG. 'Excitotoxic neuronal cell death during an oligodendrocyte-directed CD8+ T cell attack in the CNS gray matter.' Journal of Neuroinflammation 10:121 (2013). DOI: 10.1186/1742-2094-10-121. https://doi.org/10.1186/1742-2094-10-121
  5. Göbel K, Bittner S, Cerina M, Herrmann AM, Wiendl H, Meuth SG. 'An ex vivo model of an oligodendrocyte-directed T-cell attack in acute brain slices.' Journal of Visualized Experiments (96):52205 (2015). DOI: 10.3791/52205. https://doi.org/10.3791/52205
  6. Ciurkiewicz M, Herder V, Khan MA, Uhde AK, Teich R, Floess S, Baumgärtner W, Huehn J, Beineke A. 'Cytotoxic CD8+ T cell ablation enhances the capacity of regulatory T cells to delay viral elimination in Theiler's murine encephalomyelitis.' Brain Pathology 28(3):349-368 (2018). DOI: 10.1111/bpa.12518. https://doi.org/10.1111/bpa.12518
  7. Schroeter CB, Huntemann N, Bock S, Nelke C, Kremer D, Pfeffer K, Meuth SG, Ruck T. 'Crosstalk of Microorganisms and Immune Responses in Autoimmune Neuroinflammation: A Focus on Regulatory T Cells.' Frontiers in Immunology 12:747143 (2021). DOI: 10.3389/fimmu.2021.747143. https://doi.org/10.3389/fimmu.2021.747143
  8. Tsyklauri O, Chadimova T, Niederlova V, Kovarova J, Michalik J, Malatova I, Janusova S, Ivashchenko O, Rossez H, Drobek A, Vecerova H, Galati V, Kovar M, Stepanek O. 'Regulatory T cells suppress the formation of potent KLRK1 and IL-7R expressing effector CD8 T cells by limiting IL-2.' eLife 12:e79342 (2023). DOI: 10.7554/eLife.79342. https://doi.org/10.7554/eLife.79342
  9. Aydin S, Pareja J, Schallenberg VM, Klopstein A, Gruber T, Page N, Bouillet E, Blanchard N, Liblau R, Körbelin J, Schwaninger M, Johnson AJ, Schenk M, Deutsch U, Merkler D, Engelhardt B. 'Antigen recognition detains CD8+ T cells at the blood-brain barrier and contributes to its breakdown.' Nature Communications 14:3106 (2023). DOI: 10.1038/s41467-023-38703-2. https://doi.org/10.1038/s41467-023-38703-2
  10. Pareja J, Aydin S, Zbinden M, Bouillet E, Zollinger N, Theivendram V, Fahmi A, Pleskač P, Barcos S, Paas F, Kloster F, Blázquez AM, Fonta N, Merkler D, Deutsch U, Engelhardt B. 'Lack of junctional adhesion molecule (JAM)-B traps CD8 T cells in CNS border zones and ameliorates autoimmune neuroinflammation.' Acta Neuropathologica Communications 13:117 (2025). DOI: 10.1186/s40478-025-02021-z. https://doi.org/10.1186/s40478-025-02021-z